On 14/8/25 18:05, Igor Mammedov wrote:
This patch brings back Jan's idea [1] of BQL-free IO access
This will let us make access to ACPI PM/HPET timers cheaper,
and prevent BQL contention in case of workload that heavily
uses the timers with a lot of vCPUs.
1) 196ea13104f (memory: Add global-locking property to memory regions)
... de7ea885c539 (kvm: Switch to unlocked MMIO)
Signed-off-by: Igor Mammedov <imamm...@redhat.com>
Reviewed-by: Peter Xu <pet...@redhat.com>
---
v4:
improove doc comment over memory_region_enable_lockless_io()
David Hildenbrand <da...@redhat.com>
v3:
add comment for 'mr->disable_reentrancy_guard = true'
Peter Xu <pet...@redhat.com>
---
include/system/memory.h | 12 ++++++++++++
system/memory.c | 15 +++++++++++++++
system/physmem.c | 2 +-
3 files changed, 28 insertions(+), 1 deletion(-)
diff --git a/include/system/memory.h b/include/system/memory.h
index e2cd6ed126..aa85fc27a1 100644
--- a/include/system/memory.h
+++ b/include/system/memory.h
@@ -833,6 +833,7 @@ struct MemoryRegion {
bool nonvolatile;
bool rom_device;
bool flush_coalesced_mmio;
+ bool lockless_io;
bool unmergeable;
uint8_t dirty_log_mask;
bool is_iommu;
@@ -2341,6 +2342,17 @@ void memory_region_set_flush_coalesced(MemoryRegion *mr);
*/
void memory_region_clear_flush_coalesced(MemoryRegion *mr);
+/**
+ * memory_region_enable_lockless_io: Enable lockless (BQL free) acceess.
Typo "access[es]".
+ *
+ * Enable BQL-free access for devices that are well prepared to handle
+ * locking during I/O themselves: either by doing fine grained locking or
+ * by providing lock-free I/O schemes.
+ *
+ * @mr: the memory region to be updated.
"the memory region to enable [lockless accesses]"?
+ */
+void memory_region_enable_lockless_io(MemoryRegion *mr);
+
/**
* memory_region_add_eventfd: Request an eventfd to be triggered when a word
* is written to a location.
diff --git a/system/memory.c b/system/memory.c
index 5646547940..44701c465c 100644
--- a/system/memory.c
+++ b/system/memory.c
@@ -2546,6 +2546,21 @@ void memory_region_clear_flush_coalesced(MemoryRegion
*mr)
}
}
+void memory_region_enable_lockless_io(MemoryRegion *mr)
+{
+ mr->lockless_io = true;
+ /*
+ * reentrancy_guard has per device scope, that when enabled
+ * will effectively prevent concurrent access to device's IO
+ * MemoryRegion(s) by not calling accessor callback.
+ *
+ * Turn it off for lock-less IO enabled devices, to allow
+ * concurrent IO.
+ * TODO: remove this when reentrancy_guard becomes per transaction.
+ */
+ mr->disable_reentrancy_guard = true;
+}
+
void memory_region_add_eventfd(MemoryRegion *mr,
hwaddr addr,
unsigned size,
diff --git a/system/physmem.c b/system/physmem.c
index e5dd760e0b..f498572fc8 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -2900,7 +2900,7 @@ bool prepare_mmio_access(MemoryRegion *mr)
{
bool release_lock = false;
- if (!bql_locked()) {
+ if (!bql_locked() && !mr->lockless_io) {
Check @lockless_io first to avoid a call?
if (likely(!mr->lockless_io) && !bql_locked()) {
bql_lock();
release_lock = true;
}
